The invention relates to a device for the electrodeionization and demineralization of aqueous solutions (EDI) process.
EDI processes are already in use for the demineralization of aqueous solutions. Such a process is based in principle on a combination of electrodialysis, ion exchange and electrochemical dissociation of water for generating H+ and OH− ions.
A general overview over EDI processes and apparatus is described in [1].
EDI processes include cation and anion exchange membranes which are combined to a stack of preferably equidistantly arranged elements contained in a housing. The outermost membranes are each a cation exchange membrane and an anion exchange membrane. The areas between these membranes are compartments through which a liquid is conducted by way of admission and discharge openings.
Transverse to the flow direction of the parts and orthogonally with regard to the membranes, an electric DC voltage field is applied. The field is generated by way of electrodes at the ends of the stacks, the electrodes being connected to an external voltage source.
Because of their charge, the ions in the aqueous solution travel in the electric DC voltage field toward the respective counter electrodes, that is, the cations travel to the cathode and the anions travel to the anode. The cations and anions can each pass through the cation, or respectively, anion exchange membranes. In contrast, the cation exchanger membranes are almost impenetrable barriers for the anions and the anion exchanger membranes are impenetrable barriers for the cations. With the alternating arrangement of the two membrane types, compartments are formed in the DC voltage field wherein the ions are enriched (concentric compartments (KK)) and compartments in which they are depleted (Demineralization compartments (DK)).
With an EDI device, the compartments are additionally filled with bulk particles of ion exchange resins (mixed bed), wherein the individual particles (form bodies) consist either of a cation or an anion exchanger resin and the bulk consists of particle mixtures of both types of ion exchange resins. By using a mixed bed, the electric charge transport distances in the aqueous solution, whose electric resistance is substantially increased with increasing demineralization, are shortened from the distance to the closest exchange membrane with the corresponding ion charge to the distance to a particle surface of the respective anion- or cation exchanger resin fraction. Under the influence of the electrical DC voltage field, the anions and cations received by ion exchange are passed on in the respective exchange resin within the particles and are conducted to other adjacent particles. They diffuse through—depending on the ion type—the respective permeable exchanger membrane and travel in this way from the DK to a KK. It is noted in this connection that cations can move only from cation exchanger particle to cation exchanger particle and anions can move only from anion exchanger particles to anion exchanger particles. In order for ions to cross the whole DK continuous particle chains of ion exchanger particles of the same type must be present.
Basically, in [1] two variants of an EDI device are described, one variant wherein all compartments are filled with the mixed bed (see FIG. 1a), and another variant wherein only the DK is filled with a mixed bed whereas the remaining DK are not filled with mixed bed, that is, they remain empty (see FIG. 1b).
With the use of a mixed bed consequently, the ions to be removed from the aqueous solution are exchanged already by the corresponding ion exchange resin fraction of the mixed bed with H+ or OH− ions and therefore removed form the solution. The freed H+ and OH− react in the aqueous solution to form water. The product obtained in the process has a very low ion concentration and therefore low conductivity.
For performing a continuous EDI, the H+ and OH− ions consumed during the exchange must be continuously replenished. The needed H+ and OH− ions are generated by the dissociation of water under the influence of the electric field. In this way, the ion exchanger resins are continuously regenerated without the need for chemicals herefor. In principle, the EDI is therefore a highly effective and environmentally friendly demineralization process.
Utilization of EDI devices however is quite limited because of their expensive construction and the resulting high price which is caused by the high price of the membrane material and the high expenditures for the supply and discharge of the aqueous solution. Because of the statistic distribution of the cation and anion exchanger resin particles, the acceptable membrane distance is quite limited for insuring a sufficient number of ion paths by way of contacts between the same type of particles. As a result, the present EDI devices require a relatively large number of membranes.
Various attempts have been made to alleviate this problem, that is, to increase the efficiency and to reduce costs.
The publication [2] discloses the used of so-called monosphere ion exchanger resin particles (ion exchanger resin balls), that is, of ion exchangers of a uniform particle size instead of the usual ion exchanger particles with a wide particle size distribution. The use of monosphere ion exchanger resin particles results in a higher packing density of the mixed beds and improves thereby the ion transfer. However, the problem of a statistic distribution of the different ion exchanger types remains so that the optimum distance between the membranes cannot be essentially increased.
Further, in [3] an EDI device with a layered arrangement of the two ion exchanger particle fractions is described so that, in each layer, ideally either only cations or only anion exchangers are present. This results in a drastic increase of the contact area between ion exchange particles of the same type and consequently in a substantial improvement of the ion transfer within each position. This permits a substantial increase in the spacing of the membranes. But then the efficiency of the ion storage decreases with increasing layer thickness. One of the reasons herefor is that the H+ and OH− ions released during the exchange of, for example, Na+ and Cl− are not formed in the immediate neighborhood and therefore cannot combine to water without delay. In this way, these ions are not withdrawn from the ion exchange equilibrium as it is normally the case. A layer-like arrangement therefore inhibits an instant neutralization which is disadvantageous for the ion reception capability.
It is the object of the present invention to provide an EDI device wherein the efficiency of the ion reception for the demineralization of aqueous solutions is also present in stacks of EDI devices with larger membrane distances.